A male human’s reproductive system turns on at puberty,
and stays on until he dies. The pituitary gland sends a message to the
testicles (which are protected inside the sacs called
the scrotum) to begin producing testosterone
and sperm. Beginning at puberty, a healthy man produces
one thousand sperm per second twenty-four hours a day, seven days a
week. (As my grandmother would have said, this explains a lot of things.)

During the winter, a man likely produces more sperm, because
humans prefer a cooler temperature when maturing eggs. It therefore
follows that a man who bikes in tight shorts to the hot tub every day
throughout the summer after work baking pizza might find himself with
a low sperm count!

With age, a man’s sperm production and ability to have
an erection will likely decrease. Also, because of environmental toxins,
sperm counts are now decreasing at an alarming rate of 2% per year in
men of all ages. But essentially, beginning at puberty, men are fertile
all the time.

Once they’re produced, sperm go through a maturation process
in the epidydimus, which is also held in the scrotum.
The maturation process takes about two months. Just before orgasm, sperm
move from the testicles through the vas deferens (tubes
analogous to the fallopian tubes)—and collect fluid from the prostate
gland, the cowper’s gland, and the seminal
vesicle; the fluid from these glands mixes with the sperm to
create semen, which is then ejaculated through the urethra.

The chemical content of semen is similar to cervical fluid.
In a healthy man, each ejaculation contains between 250 and 350 million
sperm. Generally, it takes 36 hours to replenish a man’s sperm count.

If a man has a vasectomy, the vas deferens are cut to
prevent sperm from being ejaculated. (They’re still produced and matured
after a vasectomy; but they’re prevented from being released.) If a
man’s had a vasectomy, you can’t tell by the amount of his ejaculate:
even 350 million sperm are barely discernable to the naked eye.

Sperm are measured by their numbers, their ability to
swim, and their shape. If a man has a low sperm count, he should have
it tested again a few weeks later, because sperm counts can fluctuate
significantly in a short period of time.

It’s also important to note that the pre-ejaculate can
contain enough sperm to cause a pregnancy or a sexually transmitted
infection.